In recent years, inner-antenna plasma processing apparatuses, in which a radio-frequency antenna (or antennas) is placed in a vacuum chamber, have been developed and put to practical use. In such a plasma processing apparatus, a radio-frequency electric current is applied to the radio-frequency antenna to form an induction electric field around the radio-frequency antenna, and thereby a discharge plasma is induced in the vacuum chamber. By using this discharge plasma, an intended thin film can be formed on the surface of an object to be processed or the surface of the object can be etched.
In an inner-antenna plasma processing apparatus, a negative direct current self-bias to the plasma arises in the radio-frequency antenna because an electrostatic field is formed between the radio-frequency antenna and the discharge plasma. The bias voltage accelerates ions in the plasma and drives the ions to the radio-frequency antenna, and consequently the radio-frequency antenna itself is sputtered. Atoms and ions of the material of the radio-frequency antenna which are sputtered in this manner become mixed in the discharge plasma, which causes the problem that such atoms and ions attach to the surface of a workpiece directly or after they have once attached to the inner wall or other portions of the vacuum chamber.
In order to prevent the radio-frequency antenna from being sputtered, conventionally, the exterior of the radio-frequency antenna is shielded with a protective tube made of an insulator, e.g. quartz. Patent Document 1 discloses an inner antenna in which the circumference of a copper pipe of 6 mm in diameter is shielded with a protective tube made of quartz of 15 mm in diameter.
With the configuration disclosed by Patent Document 1, the pressure in the space between the radio-frequency antenna and the protective tube is the same as the pressure of the process gas. Hence, electric discharges occur also in that space, causing the problem that the induction electric field radiating from the radio-frequency antenna is consumed by the electric discharge in the space, making it difficult to generate plasma in the vacuum chamber, which is the target area.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. H11-317299